Those brake shoes are unobtanium. No one manufactures them anymore and they cannot be purchased anywhere. You will have to send off your current shoes to be relined, its the only option for the 15x4 size. Ill dig and try and find the specific parts number for you, but it wont be of much help.
Occasionally a NOS set pops up from an old farm shop that's being cleaned out, but it is extremely rare to ever happen.
